Fixes JENKINS-21332 https://issues.jenkins-ci.org/browse/JENKINS-21332

This pull request fixes the issue in the title by moving the fortifyclient-{Version}.jar files from the classes/ directory to a separate fortify-lib directory in the plugin's WEB-INF/ folder.

Changes in detail are:
* Moved fortifyclient-{version}.jar files to WEB-INF/fortify-lib/
* adjusted FortifyClientClassloader to load from pluginWrapper.getBaseUrl("WEB-INF/fortify-lib/fortifyclient-" + version + ".jar");
* adjusted test (needs Jenkins Test Harness now - only works with plugin loaded)